Combinations of various treatments have been suggested extensively in the art with a view to improving one or more properties of the lubricating base oil to be produced. For instance, reference is made in U.S. Pat. No. 3,256,175 to a process wherein a light distillate fraction of a crude oil is subjected to solvent extraction to give a light raffinate and a light aromatic extract, whilst a heavy distillate fraction is also solvent extracted to obtain a heavy raffinate and a heavy aromatic extract, which latter extract is at least partially subjected to a severe hydrogenation treatment and wherein at least a portion of the oil thus hydrogenated is combined with the earlier produced light raffinate. In this integrated process both the aromatic compounds and the nitrogen compounds are removed virtually complete, i.e. for more than 97%.
A combined solvent extraction-dewaxing-hydrorefining process to produce improved viscosity index lubricating base oils is described in U.S. Pat. No. 3,702,817. The hydrorefined extract is combined with the reactant stream prior to its introduction into the dewaxing stage of the process.
A combination of a catalytic dewaxing treatment to effectively reduce the pour point of a lubricating oil base stock to below -9.degree. C., followed by a catalytic hydrotreatment in order to increase the viscosity index of the lubricating oil fraction of the dewaxed oil and recovering therefrom a high viscosity index lubricating base oil stock having a pour point not higher than -4.degree. C. is described in European patent specification No. 43,681.
Also the technique of blending different lubricating base oils which have been subjected to one or more (pre)-treatments in order to improve the oxidation stability of the resulting mixture can be used advantageously, for instance as described in British patent specification No. 2,024,852.
Since the respective treatments will contribute differently to the total spectrum of properties of the lubricating base oils to be produced, as they are likely whilst improving one desired property to deteriorate others, it requires a lot of skill to produce high quality lubricating base oils of constant quality. Many times synthetic additives have to be introduced into the base oil in order to obtain a lubricating oil of acceptable quality.